Earlier quoted context omitted.
Mullvad allows Bitcoin purchases of tokens, which can then be used as a serial for the VPN that works for the time period you've purchased. Users can change tokens any time. That's probably close to what you're already doing.
Or for those less blockchain-inclined, you can just send them cash as well... https://mullvad.net/en/pricing/ ctrl+f cash (the section has no anchor)
It doesn't get any more anonymous than cash in the mail. :)